Generally, fiber optic converters are used for measuring small displacements of objects subjected to vibrations. Examples of such fiber optic converters are disclosed Poilleux U.S. Pat. No. 3,792,298, Tourret U.S. Pat. No. 3,771,873 and Brelot et al. U.S. Pat. No. 3,778,157. In these patents, fiber optic bundles are used to transmit and receive a light beam reflected off a surface for measuring the thickness or depth of a layer of a first substance. A characteristic response curve is determined from the parameters of the system. A reflected value is compared to the response curve to determine the thickness of the layer.
One disadvantage of the above fiber optic converters is that the critical angle of the interface of the first substance with a second substance is not used. The critical angle is the angle at which light rays must impact upon the interface in order for the light rays to be totally internally reflected by the interface. Another disadvantage is that these converters do not rely upon total internal reflection of light from the interface. A further disadvantage is that these converters must be mounted above the substance to be measured, thus, these converters cannot be used in blind applications.